1. Field of the Invention
The present invention relates to control and protection circuitry for refrigeration systems. More particularly, the field of the invention involves circuitry for activating and deactivating a compressor and a fan motor.
2. Description of the Related Art
Motor protection devices generally include electro-mechanical or solid state electronic devices for protection and control of motors or compressors. Conventional motor protection devices seek to regulate the current drawn by the compressor motor under various loads and conditions. By limiting the amount of current provided to the compressor motor, conventional motor protection devices protect the compressor's windings from damaging effects of high currents and high temperatures.
For example, one conventional motor protection device is a snap disc placed in series with the windings of the compressor motor. The snap disc is composed of bi-metallic layers which are in physical contact with the contact points which close the circuit. Typically, a resistive heating element which heats the bi-metallic layers is connected in series with contact points such that when the heat generated by compressor current passing through the resistive element exceeds the allowable threshold, the different metals of the bi-metallic layer expand at different rates, causing the disc to bend. This bending of the disc breaks the connection to the contact points, thereby opening the circuit to the compressor motor. Another arrangement involves placing the snap disc device in close proximity to the compressor motor so that the snap disc device may open and close in response to the temperature of the compressor motor.
Several problems may occur with a conventional snap action bi-metallic motor overload protector. One problem with the snap disc device is that the overload condition may be detected only after a significant amount of time has passed since the condition originally developed. During this lag time, significant damage to the windings of the motor can occur. Also, conventional snap disc overload protection devices are generally imprecise and non-dynamic. For instance, the temperature and current set points of a snap disc cannot account for different environmental or motor loading conditions. Finally, once the snap disc has opened the circuit to the motor windings, the restoration period of the bi-metallic device is typically excessively lengthy.
Additional motor protection devices include solid state electronic devices which control the power delivered to the compressor motor. In contrast with the electro-mechanical snap disc devices, solid state protection devices have the advantages of precision, reliability, and self-regulation. Generally, a conventional electronic protection device includes a thermostat to sense ambient and internal compressor temperatures, control logic responsive to inputs and which controls the corresponding outputs, and solid state power components which are used to apply power to the compressor motor. For instance, thermostats using thermistors as temperature sensing inputs to an electronic motor control circuit are disclosed in U.S. Pat. No. 5,231,848, "REFRIGERATOR COLD CONTROL", issued Aug. 3, 1993, which is assigned to the assignee of the present invention, the disclosures of which are explicitly incorporated by reference.
Prior art motor protection devices typically include power output stages which regulate the application of power to the compressor motor. The output of the control logic circuit drives the output power stage, either by direct electrical connection to the output stage or by indirect magnetic coupling through a relay. Both techniques offer significant advantages in accuracy, reliability, and precision over electro-mechanical methods for controlling and protecting compressor motors.
However, circuits which directly couple the control logic circuit to the power output stage suffer from problems associated with noise induced into the control logic circuit from the high current flow of the power output stage. In order to eliminate such problems, conventional solid state control circuits utilize a relay to control the activation gate of a solid state switch element, such as a SCR or TRIAC. While the use of a relay offers the benefit of electrical isolation of the control logic circuit and the power output circuit, the use of relays in compressor motor protector circuits may also be problematic. For instance, under high temperature conditions the metallic contacts of the relay may melt down and permanently close due to excessive compressor temperatures. Furthermore, the physical contacts within the relay are subject to damage from repeated wear, corrosion, metal fatigue, or other physically degrading conditions.
What is needed is a compressor motor protection device which is not as subject to noise problems or physical degradation as conventional motor protection devices.
Also needed is a motor protection device which is more accurate and precise than conventional electro-mechanical protection devices.